A proportional solenoid is a device that can freely adjust the position of a movable yoke (plunger) by controlling electric current applied to an electromagnetic coil, and proportional solenoids have been used as valve drive means and the like for flow control valves, pressure control valves, direction switching valves and the like (see Japanese Patent Application Publication No. 2004-218816 and Japanese Patent Application Publication No. H9-69432).
The structure of a flow control valve using a proportional solenoid will be described below with reference to FIG. 6.
A flow control valve 100 adjusts an opening area of a port 6 by moving a spool 3 of a spool valve 2 with a proportional solenoid 50 (valve drive means) and controls the flow rate of fluid flowing through the port 6.
The proportional solenoid 50 comprises an annular bobbin 7 and an electromagnetic coil 9 wound on the outer periphery of the bobbin 7, and a fixed yoke 10, a fixed guide 11, and a movable yoke 12 are disposed inside these electromagnetic coil 9 and bobbin 7.
The fixed yoke 10 is composed of a magnetic material and comprises a cylindrical insertion portion 10a that has an outer diameter somewhat less than the inner diameter of the bobbin 7 and a disk-shaped flange portion 10b formed at one end (right side end portion in the figure) of the insertion portion 10a and having an outer diameter almost equal to the outer diameter of the electromagnetic coil 9. The fixed yoke 10 is disposed by inserting the insertion portion 10a into the electromagnetic coil 9 and bobbin 7 and abutting the flange portion 10b against one side of the bobbin 7. The fixed yoke 10 is fixed and cannot be moved relative to the electromagnetic coil 9 and bobbin 7.
An annular convex portion 13 that protrudes in the axial direction is formed at the edge portion of a tip surface (left side end surface in the figure) of the insertion portion 10a of the fixed yoke 10, and the outer circumferential surface of the convex portion 13 is tapered at a predetermined angle such that a the outer circumferential surface inclines inwardly in the radial direction as the tip thereof is approached (side of the movable yoke 12).
The movable yoke 12 (plunger) is composed of a magnetic material and formed as a cylinder with an outer diameter somewhat less than the inner diameter of the convex portion 13 of the fixed yoke 10. The movable yoke 12 is disposed opposite to the fixed yoke 10 inside the electromagnetic coil 9 and bobbin 7, and the tip portion (right side end portion in the figure) of the movable yoke is inserted into the convex portion 13 of the fixed yoke 10. The movable yoke 12 is provided to be movable relative to the electromagnetic coil 9, bobbin 7, and fixed yoke 10 and can move in the axial direction (left-right direction in the figure) along the inner surface of the convex portion 13 of the fixed yoke 10.
The fixed guide 11 is also composed of a magnetic material and comprises a cylindrical insertion portion 11a having an outer diameter somewhat less than the inner diameter of the bobbin 7 and an inner diameter somewhat larger than the outer diameter of the movable yoke 12 and a disk-shaped flange portion 11b formed at one end (left side end portion of the figure) of the insertion portion 11a and having an outer diameter almost equal to the outer diameter of the electromagnetic coil 9. In the fixed guide 11, the insertion portion 11a thereof is inserted between the bobbin 7 and the movable yoke 12 on the opposite side from the fixed yoke 10. Further, the flange portion 11b of the fixed guide 11 is disposed by abutting against one side of the bobbin 7. The fixed guide 11 is fixed and cannot be moved relative to the electromagnetic coil 9, bobbin 7, and fixed yoke 10.
The electromagnetic coil 9, bobbin 7, fixed yoke 10, and fixed guide 11 are connected integrally by a cylindrical case 15 made from a non-magnetic material.
A member (not shown in the figure) made from a nonmagnetic material may be inserted between the convex portion 13 of the fixed yoke 10 and the insertion portion 11a of the fixed guide 11.
On the other hand, a spool valve 2 comprises a sleeve 16 having formed therein a port 6 for passing a fluid, a spool 3 disposed so that it can slide in the axial direction inside the sleeve 16, and bias means (a coil spring in the example shown in the figure) 17 for biasing the spool 3 toward the proportional solenoid 50.
A land 3a for closing the port 6 is formed in the central portion in the longitudinal direction of the spool 3, and the opening surface area of the port 6 can be adjusted by moving the spool 3 relative to the sleeve 16.
A rod portion 3b extending via a through hole 19 formed in the fixed yoke 10 of the proportional solenoid 50 is provided at one end (left end portion in the figure) of the spool 3, and this rod portion 3b is connected to the tip surface of the movable yoke 12.
In the flow control valve 100, where an electric current is applied to the electromagnetic coil 9 of the proportional solenoid 50, a magnetic circuit is formed via the fixed yoke 10, fixed guide 11, and movable yoke 12, and a magnetic attraction force proportional to the applied current is generated between the fixed yoke 10 and movable yoke 12. This attraction force acts as a thrust force Ft that biases the movable yoke 12 toward the fixed yoke 10. Where the movable yoke 12 and the spool 3 connected thereto are moved to the right by the thrust force Ft, as shown in the figure, the coil spring 17 is compressed, and a reaction force Fr is generated that biases the movable yoke 12 in the direction opposite to that of the thrust force Ft.
As a result, the movable yoke 12 and spool 3 are stopped in a position where the thrust force Ft produced by the proportional solenoid 50 is balanced by the reaction force Fr produced by the coil spring 17.
Because the thrust force Ft produced by the proportional solenoid 50 is proportional to electric current applied to the electromagnetic coil 9, the position of the movable yoke 12 and spool 3 can be adjusted by controlling the current applied to the electromagnetic coil 9. Therefore, by controlling the current applied to the electromagnetic coil 9, it is possible to adjust the position of the land 3a of the spool 3 and adjust arbitrarily the opening area of the port 6.
In such proportional solenoid 50 and flow control valve 100 using thereof, a stroke (range of reciprocating movement) of the movable yoke 12 and spool 3 is typically set to range in which the thrust force Ft is constant regardless of the position of the movable yoke 12 and spool 3.
This will be explained with reference to FIG. 7.
FIG. 7 is a graph illustrating the relationship between the thrust force Ft acting upon the movable yoke 12 and the stroke of the movable yoke 12 (a position of the movable yoke 12 where a position closest to the fixed yoke 10 is taken for zero; it can be also called a spacing between the fixed yoke 10 and the movable yoke 12) when a predetermined voltage is applied to the electromagnetic coil 9 of the proportional solenoid 50.
As follows from the figure, where the stroke of the movable yoke 12 (spacing between the fixed yoke 10 and the movable yoke 12) is small in the case the current applied to the electromagnetic coil 9 is constant, the thrust force Ft acting upon the movable yoke 12 rapidly increases from a certain point. Further, where the stroke of the movable yoke 12 increases, the thrust force Ft acting upon the movable yoke 12 rapidly decreases from a certain point.
On the other hand, in the intermediate stroke region shown by an arrow in the figure, the thrust force Ft acting upon the movable yoke 12 is almost constant, regardless of the position (stroke) of the movable yoke 12. This region is called “control range”, and usually the stroke (range of reciprocating movement) of the movable yoke 12 is set within this range.
Next, the relationship between the thrust force Ft created by the proportional solenoid 50 and the reaction force Fr created by the coil spring 17 in the flow control valve 100 shown in FIG. 6 will be explained below as a reference example based on FIG. 8.
Lines Ft1 to Ft7 in the figure indicate the thrust force Ft created by the proportional solenoid 50, and it is clear that the thrust force increases as the current applied to the electromagnetic coil 9 increases (as the line number increases).
Line Fr in the figure indicates the reaction force Fr created by the coil spring 17, and it is clear that the reaction force increases as the stroke of the movable yoke 12 decreases (as the movable yoke 12 approaches the fixed yoke 10).
In the figure, points where the thrust force lines Ft1 to Ft7 and reaction line Fr intersect (circles in the figure) are balance points of the two, and the movable yoke 12 stops in these positions.